Despite intensive research since the viral pathogen was discovered some 25 years ago, not much progress has been reported on the development of a safe vaccine that protects against human immunodeficiency virus type 1. A vaccine approach that has been abandoned because its safety cannot be guaranteed is the single vaccine candidate that provides good protection in the macaque model, a live-attenuated variant of the simian immunodeficiency virus. The attenuated virus will cause a low-grade, but persistent infection that allows optimization of viral replication kinetics over time by spontaneous virus evolution, which may increase viral pathogenicity. In this article, we discuss innovative strategies to overcome this hurdle, including the generation of "single-cycle" viruses and a conditionally replicating HIV-1 variant.

(CHRISTINE BALDERAS, GETTY IMAGES)

Vaccines that consist of a live-attenuated virus strain have proven to be very successful at inducing protective immunity against pathogenic viruses such as those causing smallpox, polio, and measles.1 Research on the development of a live-attenuated human immunodeficiency virus (HIV) vaccine has predominantly been performed in macaques that are infected with pathogenic simian immunodeficiency virus (SIV). Attenuation occurs by the deletion of several accessory functions from the viral genome, either individually or in combination.2–5 The majority of monkeys vaccinated with such deletion mutants of SIV can efficiently control replication of pathogenic challenge virus strains. However, the attenuated virus could revert to virulence and cause disease over time in vaccinated animals, especially in neonates.6–10 Similarly, some of the long-term nonprogressors of the Sydney Blood Bank Cohort, infected with a naturally attenuated HIV-1 variant with deletions in the nef and long terminal repeat (LTR) sequences eventually progressed to acquired immunodeficiency syndrome (AIDS).11 An HIV-1 Δ3 variant with deletions in the vpr, nef, and LTR sequences regained substantial replication capacity in long-term cell culture infections by acquiring compensatory changes in the viral genome.12 These results highlight the genetic instability and evolutionary capacity of attenuated SIV/HIV strains, which pose a serious safety risk for any future experimentation with live-attenuated HIV vaccines in humans. Novel strategies are needed to reconsider this vaccination approach because many other vaccine attempts have thus far failed to provide protection.13

The Safety Issue of Live-Attenuated HIV-1 Variants

The major safety problem of live-attenuated HIV/SIV vaccine strains is related to the persistent replication and consequent evolution of the attenuated virus. In combination with the error-prone replication machinery of the virus, this ongoing low-level replication may eventually lead to the appearance of fitter and more pathogenic virus variants. To improve safety, the vaccine strain can be further attenuated through additional deletions or mutations in accessory genes or regulatory elements. This further reduces the pathogenic properties of the virus, but at the same time also reduces the vaccine efficacy.14–15 As an alternative strategy to prevent evolution toward a pathogenic variant, replication of the vaccine virus should be limited to the extent and time window that is required to provide full protection. For instance, virus replication can be stopped a few weeks after vaccination by administrating antiviral drugs.16 Although this is a good research strategy for macaque studies to address whether ongoing replication of the vaccine strain is needed for protection, application in humans seems problematic because long-term virus inhibition will require continuous drug administration, and the virus may develop drug resistance. Alternatively, a virus that can execute only a single round of replication can be used as a vaccine.17–20 However, because of the limited replication, such a single-cycle virus vaccine may be less potent for inducing protective immunity. We and others previously presented an alternative approach that uses a conditionally live HIV or SIV variant.21–25 We will discuss some of these approaches in this article.